Abstract: The friction mechanism of pile-soil interface under vertical cyclic loading is the key to study the friction fatigue of piles. In order to study the cyclic shear behavior of pile-silt interface, cyclic shear tests are carried out under constant normal stiffness (CNS) using a modified shear test device. The effects of cycle numbers, cumulative displacement and normal stiffness on the friction fatigue performance and post-cyclic monotonic shear behavior are studied. The results show that the normal and shear stresses decrease rapidly in the initial 10 cycles and gradually become stable with the cycle. The soil exhibits shearing contraction, dilatation and contraction alternating phenomenon with the shear displacement direction changing in a single cycle, and the overall deformation shows a tendency of shearing contraction. The higher normal stiffness results in higher reduction rate of the normal and shear stresses in the shear interface, and the cumulative cyclic displacement for cyclic stability is smaller. The shear stress ratios from the post-cyclic monotonic and post-cyclic monotonic shear tests with the same initial normal stress are lower than those from the initial monotonic shear tests, and the post-cyclic monotonic shears from the same initial normal stress tests have low shear dilation degree, which indicates that the soil sample experiences particle crushing and soil particles become smooth due to the cyclic shearing effect. Based on the analysis of experimental data, the dimensionless cumulative displacement related to the cumulative displacement, normal stiffness and initial normal stress is proposed. The attenuation equation for the normal stress and interface friction angle with cumulative displacement is established.